1. Field of the Invention
The present invention relates to batteries. More particularly, the present invention relates to a battery charger having a multipurpose charge level sense line.
2. Description of the Related Art
Cellular radiotelephones are rapidly decreasing in size and cost. In order to accomplish these reductions, the number of parts contained in a radiotelephone are reduced. This reduces the cost of the total number of radiotelephone parts, the manufacturing costs and also reduces the weight of the unit. In order to continue these reductions in cost and weight, the radiotelephone parts count must be reduced further.
Since the battery is the portion of the radiotelephone connected to a battery charger, the radiotelephone may have contacts through the battery to the charger in order for the radiotelephone to communicate information to the charger. At least two of these contacts conduct power from the battery to the radiotelephone; one is for power and the other ground.
Another contact enables the radiotelephone to communicate the amount of charge current needed for it to both operate and charge the battery at the same time. Typically, after the battery is fully charged, the charge current is reduced to zero or to a very small "maintenance charge" level. This contact allows the radiotelephone to inform the charger that it is turned on and therefore requires more current than the normal maintenance charge.
Yet another contact connects the battery's thermistor to the charger so that the charger knows when the battery is fully fast charged. As the battery finishes fast charging, its temperature increases since a fully charged battery converts additional charge current into heat. This increased temperature decreases the resistance of the thermistor. The charger knows that the battery is charged by monitoring for this characteristic change in resistance.
All of these contacts add weight and complexity to the radiotelephone. There is a resulting need for a multiple purpose connection between the radiotelephone and the battery to reduce the weight, cost, and number of components in a radiotelephone.